1. Field of the Invention
The invention relates to a control apparatus and a control method for an internal combustion engine configured to execute air-fuel ratio control.
2. Description of Related Art
Generally, in an internal combustion including an exhaust gas control system that uses a catalyst, in order to highly efficiently remove pollutants from exhaust gas using the catalyst, it is necessary to control the mixing rate between air and fuel in a mixture that is burned in the internal combustion engine, that is, to control the air-fuel ratio. In order to execute the control of the air-fuel ratio, an air-fuel ratio sensor is provided in an exhaust passage of the internal combustion engine, and a feedback control is executed so that the air-fuel ratio detected by the sensor becomes equal to a predetermined target air-fuel ratio.
Generally in a multi-cylinder internal combustion engine, the air-fuel ratio feedback control is executed by using the same control amount for all the cylinders. Therefore, despite execution of the air-fuel feedback ratio control, the actual air-fuel ratio sometimes varies among the cylinders. In such a case, if the degree of variation in the air-fuel ratio is a small degree, the variation in the air-fuel ratio can be absorbed by the air-fuel ratio feedback control, and pollutants in exhaust gas can be removed by the catalysts. Thus, small degrees of the variation in the air-fuel ratio do not affect the exhaust emissions, and therefore do not cause any particular problem.
However, if the air-fuel ratio greatly varies among the cylinders due to, for example, failure of the fuel injection system of at least one cylinder or the valve actuation mechanism for the intake valves, etc., the exhaust emission quality deteriorates, thereby causing problems. Various methods and apparatus that detect such a large variation in the air-fuel ratio as to deteriorate exhaust emissions have been proposed.
For example, in an internal combustion engine disclosed in Japanese Patent Application Publication No. 2010-112244 (JP 2010-112244 A), it is firstly determined whether the air-fuel ratios of the cylinders of the internal combustion engine are in an imbalanced state on the basis of a computed value regarding the air-fuel ratio feedback control. In this internal combustion engine, a main air-fuel ratio feedback control is executed on the basis of a result of detection by an A/F sensor provided upstream of an exhaust gas purification catalyst in an exhaust passage, and a subsidiary air-fuel ratio feedback control is executed on the basis of a result of detection by an O2 sensor provided downstream of the exhaust gas purification catalyst. When the average value of computed values in the subsidiary air-fuel ratio feedback control exceeds a usual value, it is determined that the air-fuel ratios of the cylinders are in the imbalanced state. Furthermore, in the internal combustion engine of JP 2010-112244 A, when it is thus determined that there is an air-fuel ratio abnormality among the cylinders, a process of shortening the fuel injection duration for each cylinder by a predetermined amount of time is executed, and a cylinder in which a misfire is caused by the process is specifically determined to be a cylinder that causes an air-fuel ratio imbalance.
In the technologies of the Japanese Patent Application Publication No. 2010-112244 (JP 2010-112244 A) and the like, it is possible to detect inter-cylinder air-fuel ratio imbalance in so-called multi-cylinder internal combustion engines that have a plurality of cylinders, and particularly, it is possible to detect that an abnormal lean deviation is occurring in an abnormal cylinder, that is, the air-fuel ratio is deviated to the lean side in an abnormal cylinder. If an ordinary air-fuel ratio feedback control is simply executed when such an abnormal lean deviation is occurring, the amount of fuel injection for all the cylinders becomes large. Therefore, for example, in the case where the degree of abnormal lean deviation is great and an exhaust gas purification catalyst provided in the exhaust passage is a so-called three-way catalyst, the exhaust air-fuel ratio of a normal cylinder may depart to the rich side from a high-efficiency process region for the exhaust gas purification catalyst, and consequently, the catalytic conversion rate of the hydrocarbon component may decline. The possibility of the occurrence of such a problem increases particularly in the case where an abnormal lean deviation occurs in a cylinder that discharges the exhaust gas that strongly influences the air-fuel ratio sensor provided in the exhaust passage of the internal combustion engine.